Check out these Chickadee nest tube plans. Just about all chickadee researchers use something like this, allowing them to monitor and band the young safely. Because producing PVC pipe is so polluting, it's best to find used or scrap materials. Make sure to provide ventilation so moisture doesn't build up inside and so no potentially toxic fumes could build up in there. Also, pack it full of wood shavings. We chickadees LOVE fixer-uppers, and feel most at home when we get to do our own excavating.